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Baseline study

Research on Antarctic sea urchins makes first long-term measurement of ocean pH


Talk about a bad acid trip.

By the end of the 21st century, if humans keep pumping carbon dioxide (CO2) into the atmosphere as prodigiously as recent decades, the world’s oceans may become a very inhospitable place for shell-building organisms, starting with the polar regions.

That has drawn a team of researchers to McMurdo Station in Antarctica for the last two field seasons to gauge the effects of ocean acidification on the embryos and larvae of a purplish sea urchin called Sterechinus neumayeri.

“We didn’t know what the tolerances of marine invertebrates would be because the way they’re stereotyped is that they have a very narrow ocean temperature and chemistry range, so we wanted to explore that in the lab,” explained Gretchen Hofmann,an eco-physiologist and professor at the University of California, Santa Barbara.

“They’re a little more resilient than we thought,” she added. At least based on the first battery of experiments by the researchers, who spawn the critters in McMurdo’s laboratory aquarium for various experiments that mimic high levels of atmospheric CO2 on seawater.

Sea urchins and many other marine invertebrates, from corals to clams, use the calcium carbonate minerals of calcite or aragonite to construct their shell coverings or skeletons. This requires supersaturating concentrations of the carbonate ion in seawater.

Normally that’s not a problem. However, it turns out that oceans serve as a great sink for CO2, absorbing up to a third of it from the atmosphere. But it comes at a cost by altering the seawater chemistry and turning it more acidic. As ocean pH falls, so does the concentration of the carbonate ion.

The pH level, measured in units, is a calculation of the balance of a liquid’s acidity and alkalinity. The lower a liquid’s pH number, the higher its acidity. The worse case scenarios for future CO2 levels by the year 2100 translate to dangerously low pH levels, according to the 2007 Intergovernmental Panel on Climate Change (IPCC) report.

The spiky sea urchin found on the seafloor around Antarctica might be one of the first organisms to feel the effects of ocean acidification due to the cold polar waters in which it lives. That’s because CO2 more readily dissolves in colder water, meaning polar oceans would acidify first.

And there’s already evidence that the open ocean pH has dropped. Before the Industrial Revolution, ocean pH was nearly 8.2. It has dropped to about 8.1. At the end of the 21st century, under the “business as usual” model of burning fossil fuels at current rates, the IPCC report says the pH could plunge down to 7.8, which many researchers consider a threat to marine life.

In fact, research led by scientists at Stanford University in 2010 suggested that the world’s biggest extinction event 250 million years ago was caused by a combination of high levels of CO2 in the atmosphere and higher acidity in the oceans. The so-called Permian-Triassic extinction wiped out 90 percent of marine species and about three-quarters of land species.

However, there are still many uncertainties surrounding ocean acidification. Researchers have only really begun to test how different marine organisms may respond to higher acidity. An even bigger unknown is the exact pH of the ocean in different regions around the world. That 8.1 pH number is just a rough estimate.

Principal investigator on the Antarctic sea urchin project, Hofmann is also collaborating on more broad-scale studies, including another National Science Foundation-funded program called Ocean Margin Ecosystem Group for Acidification Studies (OMEGAS). OMEGAS is a consortium of scientists studying and monitoring ocean acidification along the west coast of the United States.

She was also the lead author of a paper published late last year in the journal PLoS ONE that monitored the pH levels at 15 locations around the world, including seawater in the Antarctic, as well as temperate locations and the deep ocean.

“We were able to illustrate how parts of the world’s oceans currently have different pH, and thus how they might respond to climate changes in the future,” Hofmann had explained in a press release from UCSB.

The scientists employed a new technology developed by Todd Martz, a marine chemistry researcher at Scripps Institution of Oceanography at University of California, San Diego, called a SeaFET. The sensors are suspended just off the seafloor to a mooring cage and held up by buoys. They were tested for the first time in Antarctica during the 2010-11 field season.

The instruments continuously and autonomously monitor pH, providing important baseline data from which scientists can monitor future changes in ocean acidity, according to Hofmann.

“It’s a little device that’s been giving us some really important data. We’re really stoked to be using it down here, because this is the first time we’ve ever had any of these kinds of numbers for places under the sea ice,” Hofmann said.

So far, she and her colleagues have found widespread variations between sites. Coastal areas off the western United States experience regular 24-hour fluxes in pH levels that would be considered highly corrosive to the organisms that live in those regions, but there are no obvious ill effects. In Antarctica, the pH seems to be fairly steady.

“This is something we didn’t know,” Hofmann said. “It’s really hard to say what’s acidic and what is not. On the coast, we do have acidic events that are due to other processes.”

Just what’s acidic to the Antarctic sea urchin is something Hofmann’s team is exploring in a variety of ways, from the effects on the organism’s metabolism down to the cellular level.

McMurdo divers Rob Robbins and Steve Rupp collected adult sea urchins under the ice from an area near Cape Evans on Ross Island where the critters are particularly productive, according to UCSB graduate student Lydia Kapsenberg, who works in Hofmann’s lab.

The sea urchins were then spawned in the lab, and about 160,000 larvae put into white plastic buckets immersed in a tank of seawater chilled to an ambient minus 1.8 centigrade.

Different colored lines snake from the ceiling of the aquarium lab carrying CO2 to the various buckets. Blue-colored gas lines pump in carbon dioxide at 395 parts per million (ppm), roughly today’s current atmospheric conditions. A yellow line carries gas at 600 to 700ppm, and the red line is at the extreme 1,000ppm for end-of-the-century predictions.

Pauline Yu, an NSF Office of Polar Programs postdoctoral research fellow working on the project, is in charge of the respirometry experiments. She explained that the team is interested in how ocean acidification might affect the organism’s growth, and metabolism has an important impact on that. Previous research on sea urchins has provided the group with the animal’s baseline metabolic state.

“We want to see if there is a metabolic rate response in the larvae in response to the high CO2 levels,” Yu said. “They responded by being a little smaller.”

Meanwhile, Kapsenberg oversees a different experiment that adds another stress — heat. An aluminum heat block with holes drilled in it for vials warms up the seawater as high as 20 degrees centigrade. About 20 percent of the developing larvae survive the extreme heat shock, she said.

Climate change won’t work that fast, but some of even the most conservative estimates predict serious changes to both ocean acidity and temperature in the coming decades and centuries. Evolution is often considered a slow process, but Hofmann noted that there are instances of “rapid evolution.”

Whether species like the polar sea urchin have such an evolutionary elasticity is another matter.

“The environmental conditions that something gets used to and lives at and adapts to over long period of time shapes its ability to respond to future change,” Hofmann said.

NSF-funded research in this article: Gretchen Hofmann, University of California, Santa Barbara, Award No. 0944201.

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Curator: Peter Rejcek, Antarctic Support Contract | NSF Official: Winifred Reuning, Division of Polar Programs